Поиск дочерних окон

<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
Private Shared Function PostMessage(ByVal hWnd As IntPtr, ByVal Msg As UInteger, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As Boolean
End Function
Private Declare Function FindWindow Lib "user32.dll" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
Private Declare Function FindWindowEx Lib "user32.dll" Alias "FindWindowExA" (ByVal hWndParent As IntPtr, ByVal hWndChildAfter As Integer, ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
 
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim Window = FindWindow(vbNullString, "Главное окно")
Dim hWnd = FindWindowEx(Window, 0, vbNullString, "ОК") 
End Sub

System.Windows.Automation